1. Field of the Invention
The present invention relates to laminated magnetic cores and, more particularly, to a method of forming a laminated magnetic core with sputter deposited and electroplated layers.
2. Description of the Related Art
An alternating current input to a coil generates an alternating magnetic field. One way to increase the strength of the magnetic field is to wrap the coil around a magnetic core, which is a structure that is formed from a material with a high magnetic permeability relative to air. In semiconductor fabrication processes, magnetic cores are commonly formed as thin-film structures by electroplating or sputter depositing a magnetic material.
In a typical magnetic core electroplating process, a seed layer is first sputter deposited on a substantially-planar, non-conductive surface, followed by the formation of a plating mold (e.g., a patterned photoresist layer) on the seed layer. After this, a magnetic material, such as permalloy, is electroplated to form a magnetic core on the region of the seed layer that was exposed by the plating mold. The plating mold and the seed layer lying below the plating mold are then removed.
In a typical magnetic core sputter depositing process, a target of magnetic material, such as a permalloy target, is bombarded with energetic particles, such as ions. The energetic particles, in turn, cause the target of magnetic material to eject atoms, such as permalloy atoms, which are then deposited on a substantially-planar, non-conductive surface.
One significant advantage of electroplating over sputter depositing a material is that the deposition rate of an electroplated material is substantially greater than the deposition rate of a sputter deposited material. For example, the deposition rate of an electroplated material can be upwards of 1 μm per minute, whereas the deposition rate of a sputter deposited material can be upwards of 4 μm per hour.
Thus, to form a magnetic core that is 4 μm thick, it can take an hour or more to sputter deposit the material compared to 4 minutes or more to electroplate the same amount of material. The difference in deposition rate, however, is not the only consideration when forming a magnetic core.
Another important consideration is the frequency of the alternating current that is to be input to the coil. As the frequency of the alternating current increases, eddy currents generated within the coil reduce the amount of alternating current that can flow through the center of the coil, thereby generating an alternating current resistance.
In addition, as the frequency of the alternating current increases, eddy currents generated within the magnetic core also substantially increase the alternating current resistance of the coil. Further, as the thickness of the magnetic core increases, the magnitude of the alternating current resistance from the magnetic core increases at a much greater rate.
Thus, the higher the frequency, the greater the alternating current resistance becomes. For example, given a magnetic core that is 4 μm thick, an alternating current with a frequency of 10 MHz experiences a substantially larger alternating current resistance than does an alternating current with a frequency of 100 KHz.
To reduce the alternating current resistance that results from eddy currents in the magnetic core, laminated magnetic cores are typically utilized. A laminated magnetic core is a type of magnetic core that has layers of magnetic material and layers of non-conductive material which are arranged such that each vertically adjacent pair of magnetic layers is separated by a non-conductive layer.
Laminated magnetic cores are commonly formed by sputter depositing an initial layer of magnetic material onto a substantially-planar, non-conductive surface. After the initial layer of magnetic material has been sputter deposited, a layer of non-conductive material followed by a layer of magnetic material are repeatedly sputter deposited until the required core thickness has been obtained. The last sputter deposited layer forms a stack of layers. The stack of layers is then masked and etched to form a laminated magnetic core.
With laminated magnetic cores, each layer of magnetic material in the stack is thin enough to substantially reduce the eddy currents generated in the magnetic layer at the alternating current frequency. As a result of substantially reducing the eddy currents in the magnetic core, the alternating current resistance of the coil at higher frequencies can be substantially reduced.
However, one drawback of sputter-deposited laminated magnetic cores is that it takes a significant amount of time to form a laminated magnetic core (e.g., two and one-half hours or more to form a core 10 μm thick). Another drawback is that each layer in a sputter-deposited laminated magnetic core increases the stress of the stack. Thus, thick laminated magnetic cores (e.g., 10 μm thick or thicker) formed by sputter deposition have significant stress levels which, in turn, substantially increase the likelihood of device failure.
One approach to reducing the time required to form a laminated magnetic core begins by forming a first electroplated magnetic structure on a non-conductive structure in a conventional manner (e.g., deposit a seed layer on the non-conductive structure, form a plating mold on the seed layer, electroplate, and remove the plating mold and the seed layer lying below the plating mold).
After the first electroplated magnetic structure has been formed, a polymer structure, such as an SU-8 structure, is formed to cover the first magnetic structure. After the polymer structure has been formed, a second electroplated magnetic structure is formed in a conventional manner on the polymer structure (e.g., deposit a seed layer on the polymer layer, form a plating mold on the seed layer, electroplate, and remove the plating mold and the seed layer lying below the plating mold). The process of forming a polymer layer followed by the conventional formation of an electroplated magnetic structure then continues until the required thickness of the magnetic core has been obtained.
One drawback of this approach is that the approach utilizes a different plating mold to form each magnetic layer. The plating molds, in turn, are typically implemented as patterned photoresist layers, which are relatively expensive to form in a semiconductor fabrication process. Thus, although faster, this approach is quite expensive. As a result, there is a need for a fast inexpensive approach to forming laminated magnetic cores.